When sewage is being conveyed to a sewage treatment works or when it is undergoing its initial phase of primary settlement, there may be prolonged periods where it is not in direct contact with air. During such periods, bacteria present will use up the available dissolved oxygen. In the typical sewerage system, these periods can occur when the sewage is at a comparatively high temperature in large gravity sewers where the air surface to volume ratio is low, or in pumped sewers or rising mains.
Sewage that has become anaerobic can form malodorous compounds and generate hydrogen sulfide gas which, when liberated from the sewage, can be oxidized by other bacteria to sulfuric acid of sufficient strength to attack concrete and metal fittings. The presence of a large proportion of septic sewage arriving at a sewage treatment works can also have an inhibitory effect on the bacterial reactions in the sewage treatment plant itself.
To prevent septicity of sewage several methods are employed, for example, the addition of chlorine or sodium hypochlorite to kill some of the bacteria present and inhibit the respiration rate. Similarly, iron salts may be added to fix the sulfide ions present therein as black iron sulfide, or oxygen-containing compounds, such as sodium nitrate, are added as an oxygen source for the facultative bacteria present. These methods are not always effective and have the disadvantage of adding chemicals and sometimes heavy metals to the sewage.
One well known method of treatment to prevent or reduce septicity of sewage involves the use of industrial oxygen. This is added to the sewage in such concentration as will balance the demand caused by the bacterial respiration for the period that the sewage is contained under anaerobic conditions. The method is straightforward and environmentally friendly but has a limitation in that under some conditions, such as high temperatures and/or long retention times, it is impossible to dissolve sufficient oxygen within the sewage liquor. Also, if the gas added exceeds the solubility limit of the sewage, the undissolved gas can adversely affect pipe pumping capacities etc.
The treatment method provided in accordance with the present invention substantially reduces and possibly eliminates the above mentioned problem.